This research addresses the question of how the genes of a multicellular organism specify the development and final form of the organism. This question is addressed by focusing on the genetic basis of the development of a specific, well-defined, dispensible structure in the soil roundworm Caenorhabditis elegans. The structure that will be studied in the copulatory organ of the adult male tail. This structure comprises 164 male- specific cells that arise by post-embryonic divisions of 12 precursor cells. Genes will be identified that determine both the production of the male tail-specific cells and the coordinated differentiation of those cells into the final adult structure. Genes will be identified by isolating mutants with abnormal male tails (mab mutants). Mutant isolation will be accomplished by screening mutagenized cultures with the dissecting microscope. Mutations will be placed into complementation groups and mapped with respect to known genes. Mutant phenotypes will be analyzed in detail with the compound microscope. Mutations are especially sought that affect a particular set of easily-observed sublineages within the tail. These sublineages are responsible for producing repetitive structures called rays. Electron microscopic, immunocytochemical, and laser ablation techniques will be used to determine whether another prominent structure, the fan, is also a product of these sublineages. Mutations that affect the fan and rays will be analyzed at the level of cell lineage. The aim is to identify genes that specify cell fates in the ray sublineages. Basic studies of normal development in accessible model systems, such as those described here, will aid in the understanding of disrupted processes in developmental and neoplastic diseases.